Through almost the entire time that the first Human Genome was being mapped and sequenced, a number of companies (that are now part of GE Healthcare) actively developed and provided essential enzymes, dyes, methods, and instruments to the effort. Now that this effort is complete, NHGRI has recognized that a promising new demand for sequence information will emerge once the cost of obtaining that information is decreased by 1000-fold or more. While a number of promising approaches to collect sequence information quickly and inexpensively are under investigation, most of those approaches involve a complex interplay between dye and nucleotide chemistries, enzymology, instrumentation, and data analysis. GE proposes to use existing enzyme and dye-tagged nucleotide resources in a new way that will simplify the fundamental, front-end chemistry of massively parallel sequencing-by-synthesis. GE's proposed method uses the natural catalytic cycle of DNA polymerase to capture just a single nucleotide on an immobilized primer/template. The nucleotide is captured prior to the chemical reaction step of the polymerization cycle, so it can be tagged with a dye attached to the pyrophosphate, a part that eventually is discarded. However, prior to completing the reaction cycle, the tagged nucleotide is identified using a fluorescence scanner that will scan hundreds of thousands of similar molecules at one time. When scanning is complete, the synthesis cycle is finished by simply adding a buffer containing divalent metal ion. Previous GE research efforts have identified native polymerases that incorporate such tagged nucleotides at least 15- 20% as rapidly as unmodified dNTPs. Thus, this new chemistry will simplify the overall system requirements for sequencing-by-synthesis, permitting much more flexible systems and enabling significantly longer read-lengths.